This invention relates to a cathode ray tube, and particularly to a color cathode ray tube having with an improved frame mask assembly and which is long durable.
Generally a color cathode ray tube comprises a shadow-mask assembly which is constituted by, as shown in FIG. 1, a shadow mask 1 and a mask frame 2. The shadow mask 1 is welded to the mask frame 2. The mask frame 2 is attached to a panel 3 by means of a frame holder (not shown) in such way that the shadow mask 1 is positioned to face a phosphor screen 4 formed on the panel 3 at a predetermined distance. The panel 3 is fused to a funnel 5, and an electron gun assembly 6 is disposed in the funnel 5. The envelope constituted by the panel 3 and the funnel 5 is then evacuated, whereby the cathode ray tube is completed.
Usually, the shadow mask 1 is made of low-carbon steel, and the mask frame 2 of mild steel. The surface of the mask 1 and frame 2 is blackened or oxidized so as to absorb electron rays and to suppress emission of secondary electrons and rusting during the manufacture of the cathode ray tube. A black oxide film, if formed on the mask 1 and frame 2 by blackening treatment, often peels off when the shadow mask assembly receives impact or vibration during the manufacture process of the cathode ray tube, perhaps for the following reason.
X-ray diffractiometry ascertains that such a black oxide film consists of 80 .about.60 wt. % of Fe.sub.2 O.sub.3 and 20 .about.40 wt. % of Fe.sub.3 O.sub.4. Oxidation is generally thought to proceed along with the reaction of O.sub.2 with Fe diffusing in the oxide film and reacting the surface thereof. Thus, the farther the oxidation goes, the more pores are formed in the interface portion between the substrate and the oxide film. As a result, there is formed a porous Fe layer which is mechanically very weak. The black oxide film, though considered relatively dense, is porous. Foreign matters such as H.sub.2 O and Cl ions, if put on the black oxide film, would therefore quickly reach the interface between the substrate and the black oxide film during a heating process, for example stabilizing process. Consequently, red rust having a little adhesion is formed in the porous Fe layer which is mechanically very weak. Thus, the black oxide film is quite liable to peel off.
If the black oxide film peels off in the above-mentioned manner and is broken into pieces, the pieces may clog the apertures of the shadow mask, thereby degrading the quality of a color image or reducing the breakdown voltage resistance of the cathode ray tube.